The present invention relates to a steel wire used for reinforcement of rubber articles or the like having high strength and excellent ductility and a method of manufacturing the same.
In a conventional manufacturing process for a steel wire used for reinforcement of rubber articles such as steel radial tires or high pressure hoses, a high carbon steel rod containing about 0.70xcx9c0.90% in weight of carbon is drawn to an intermediate diameter and subjected to a heat treatment and brass plating to form a steel wire material, and then the steel wire material is drawn to the final diameter. When the steel wire thus obtained is used for reinforcement of a rubber article, the steel wire is embedded in non-vulcanized rubber in a form of a single wire or a steel cord formed by a plurality of the steel wire twisted together, and then heated to achieve vulcanization of the rubber and adhesion between the steel wire and the rubber.
Recently, a steel wire of higher strength is strongly desired with growing demand for conservation of energy and natural resource. In order to produce a steel wire of higher strength by the conventional manufacturing process described above, it is necessary to increase the amount of drawing performed on the steel wire material. However, when the amount of drawing is increased, ductility of the steel wire is deteriorated to cause frequent wire breakage in processing or poor durability in use. And in some cases, deterioration of ductility particularly at the surface layer of the steel wire can be a ruling factor on possible amount of drawing or achievable strength. This phenomenon is due to the fact that the drawing strain can concentrate more easily at the surface layer than can at the internal portion of the steel wire, making the surface layer become unable to withstand further drawing earlier than the internal portion. Moreover, deterioration of ductility at the surface layer can be aggravated by age hardening or poor lubrication due to heat generated by friction with drawing die. In order to overcome such problems in ductility, some improvements in drawing technique have been proposed.
Among such improvements in drawing technique, one approach is to control age hardening of steel wire by suppression of heat generation during drawing. For example, JP-A 8-24938 discloses a manufacturing method of a high tensile strength steel wire having such ductility that the steel wire can be given a large amount of torsion until it breaks when the wire is twisted to one direction wherein drawing at the final die is carried out with control of heat generation by limitation of friction coefficient and application of a skin pass with reduction of 2xcx9c11%.
Further, JP-A 8-218282 discloses a high tensile steel wire having torque reduction ratio of less than or equal to 7% in a torsion-torque test in which a steel wire is twisted to one direction and then twisted to the opposite direction. As a manufacturing method of such a steel wire, JP-A 8-218282 also discloses a drawing method wherein (1) drawing resistance is reduced by using dies of shorter bearing, (2) skin pass is adopted for the final drawing using double die, (3) dies with sintered diamond nib are used at several passes located downstream in order to reduce drawing force, and (4) temperature of lubricating fluid is maintained low.
However, even though a steel wire of less age hardening degree can be produced, the above drawing methods do not give essential improvement as to the concentration of strain at the surface layer, rather, causing more concentration of strain at the surface layer in case of applying excessively low reduction per die. Therefore, good ductility of a steel wire shown immediately after drawing can be largely deteriorated by preforming in cabling or by progression of age hardening due to heating in rubber.
Conventionally, ductility of a steel wire has been evaluated by value of breaking torsion which is defined as an amount of unidirectional torsion applied to a steel wire until the steel wire is broken, or by considering both value of breaking torsion and form of fracture. Another evaluating method is adopted in JP-A 8-218282 wherein ductility of a steel wire is evaluated by torsion-torque curve obtained in a torsion test in which a steel wire is twisted in one direction for a certain number of turns and then twisted in the opposite direction until the steel wire is broken.
However, steel wires showing good results in above evaluating methods do not always maintain good ductility after preforming such as cabling or after heat aging followed by preforming, and improvement in durability of rubber articles reinforced by such steel wires is not assured.
Generally, in production of a steel cord for reinforcement of rubber articles, steel wires are preformed so as to have minimum radius of curvature ranging from about 10 to 150 times their diameter. Particularly, production of such steel cords listed below comprises such a severe preforming that a steel filament is preformed so as to have minimum radius of curvature ranging from about 10 to 60 times its diameter. Therefore, when a conventional steel wire is used as a filament of such steel cords, ductility is considerably deteriorated by the severe preforming and further deteriorated largely by heating in rubber.
(1) A steel cord having so-called xe2x80x9copen structurexe2x80x9d comprising largely preformed steel filaments.
(2) A steel cord comprising a steel filament preformed into a polygonal spiral shape or wavy shape.
(3) A steel cord of core and sheath structure having a core comprising a steel filament formed into a wavy shape.
Another approach for controlling the deterioration of ductility accompanying with increase in tensile strength is to make distribution of strain in a steel wire developed by drawing more uniform so as to control deterioration of ductility in the surface layer where the strain reaches maximum. For example, JP-A 7-305285 discloses a method for manufacturing a steel wire wherein:
(1) reduction of each die used for drawing where drawing strain xcex5 is less than 0.75 is set within a range between (22.67 xcex5+3)% and 29%, wherein xcex5=2ln(d0/d), d0 is diameter in mm of steel wire material before drawing, and d is diameter in mm of steel wire after passing the die,
(2) reduction of each die used for drawing where xcex5 is not less than 0.75 and not more than 2.25 is set within a range between 20% and 29%; and
(3) reduction of each die used for drawing where xcex5 is more than 2.25 is set within a range between (xe2x88x926.22 xcex5+43)% and (xe2x88x925.56 xcex5+32.5)%.
By this method, substantial drawing strain at the surface area can be controlled, but controlling effect on age hardening due to heat generated by drawing is insufficient, and economical production becomes difficult with increasing drawing speed because of frequent wire breakage in cabling or drawing process.
In view of above problems of prior art, it is an object of the present invention to provide a steel wire having such a excellent ductility that the steel wire hardly breaks in cabling and little deteriorates by preforming or age hardening after preforming. And another object is to provide a method for economically manufacturing such a steel wire.
After various experiments and studies, the inventors found that very important points for achievement of above objects are;
(1) ductility of surface layer of a steel wire should be evaluated and regulated by a specially arranged repeated torsion test, and
(2) optimization of reduction per die at the final die is necessary as well as uniform distribution of strain induced by drawing for economical manufacturing of such a steel wire.
The present invention has been done based on the important points mentioned above and includes following aspects in which [1]xcx9c[4] relate to a steel wire having excellent ductility which little deteriorates by preforming or by age hardening after preforming, and [5]xcx9c[7] relate to a method of manufacturing such a steel wire economically.
[1] A steel wire having a diameter ranging from 0.10 mm to 0.40 mm obtained by subjecting a high-carbon steel wire material having a carbon content ranging from 0.70% to 0.90% in weight to heat treatment and wire drawing, characterized in;
that tensile strength TS (N/mm2) of the steel wire satisfies following formula
TSxe2x89xa72250xe2x88x921450 log Dxe2x80x83xe2x80x83(1) 
wherein D is the diameter of the steel wire in mm and log means common logarithm,
and that repeated torsion value RT (turns/100D) of the steel wire, which is defined as sum of forward twisting and reverse twisting given until a crack is formed on a steel wire in a test wherein a steel wire is subjected to a repetition of forward twisting equivalent to 3 turns per 100D and reverse twisting to the original state, satisfies following formula.
log RTxe2x89xa72xe2x88x920.001{TSxe2x88x92(2250xe2x88x921450 log D)}.xe2x80x83xe2x80x83(2) 
[2] A steel wire having above characteristics wherein tensile strength TS (N/mm2) satisfies following formula.
TSxe2x89xa72750xe2x88x921450 log Dxe2x80x83xe2x80x83(3) 
[3] A steel wire of less concentration of strain at the surface layer having repeated torsion value RT not less than 60% of RT of the same steel wire the surface layer of which has been removed by the amount equivalent to 10% of total volume.
[4] A steel wire especially suitable for reinforcement of rubber articles and having above characteristics wherein breaking torsion value, which is defined as an amount of twisting to one direction subjected to a steel wire until the steel wire is broken, is not less than 20 turns per 100D when the steel wire has been given such a preforming that the steel wire has minimum radius of curvature of 10 to 60 times its diameter and embedded in rubber and taken out from the rubber after vulcanization.
[5] A method of manufacturing a steel wire having above characteristics by drawing a high-carbon steel wire material after heat treatment, characterized in that the drawing is carried out according to following conditions;
{circle around (1)}reduction per die is set form (22.67 xcex5+3)% to 29% for dies at which xcex5 is less than 0.75,
{circle around (2)}reduction per die is set from 20% to 29% for dies at which xcex5 is not less than 0.75 and not more than 2.25,
{circle around (3)}reduction per die is set from (xe2x88x925.56 xcex5+32.5)% to (xe2x88x926.22 xcex5+43)% for dies at which xcex5 is more than 2.25 except for the final die,
{circle around (4)}reduction per die is set from 4% to (xe2x88x928.3 xcex5+40.6)% for the final die, and
{circle around (5)}xcex5 at the final die is set from 3.0 to 4.3,
wherein xcex5 is drawing strain expressed by a formula xcex5=2ln(d0/d) (4), d0 is diameter of the steel wire material in mm before drawing, d is diameter of the steel wire in mm after passing through a die, and ln means natural logarithm.
[6] A method of manufacturing a steel wire which enables economical manufacturing of super high tensile steel wire, wherein xcex5 at the final die is set from 3.5 to 4.2 in the method of manufacturing a steel wire described above.
[7] A method which makes above method of manufacturing a steel wire more effective, wherein a bending operation with tension is applied to the steel wire drawn through the final die.